Compact silicon-on-insulator-based multimode interference coupler with bilevel taper structure

A novel compact silicon-on-insulator- (SOI-)based multimode interference (MMI) coupler with bilevel taper structures was designed. The MMI section and the S-bend sections of the input-output waveguides are deeply etched. The input-output waveguides connecting to single-mode fibers or other photonic light circuits are etched shallowly to yield single-mode operation. A bilevel taper is introduced in the transition region between the shallowly and deeply etched regions. It is predicted theoretically that this design will not only improve the quality of the self-imaging in the MMI section but will also make the structure compact. Both the excess loss and the nonuniformity of the MMI coupler are reduced. By use of a three-dimensional beam propagation method, the performance of a 1 x 4 MMI coupler based on a SOI is simulated as a numerical example of the novel design. The simulated nonuniformity and the excess loss are approximately 0.0285 and 0.2 dB, respectively.     

A 2×2 multimode interference coupler with exponentially tapered waveguide

An exponentially tapered structure is introduced into multimode interference (MMI) devices. Compared with a parabolically tapered structure, which has been successfully used in MMI devices, this structure can further reduce the length of these devices. The performances of the 2×2 MMI coupler with exponentially tapered structure, such as the optical transmission, the splitting ratio, the wavelength response and the fabrication tolerance, are investigated by the 2D finite difference beam propagation method. Results show that the exponentially tapered MMI coupler exhibits a similar property to that with a parabolically tapered structure except for the splitting ratio. The exponentially tapered structure can offer a possible application in MMI couplers with a free choice of the splitting ratio.     

Novel approach for design of low index contrast multimode interference devices

Self-imaging theory is widely accepted as a good method in designing multimode interference (MMI) couplers, but it is also true that self-imaging theory is not suitable for low-contrast structures. An improved self-imaging theory is proposed in this paper for the optimal design of low-contrast 1 × N MMI couplers. The average effective width of the MMI waveguide and the average effective propagation constant of the MMI waveguide are used in the improved self-imaging theory. An approach is given to find the average effective width. We use this approach in the optimal design of a 1?×?4 silica MMI coupler, and the results show that the improved self-imaging theory is more accurate than conventional self-imaging theory for low-contrast structures.     

Design of phased-array wavelength division multiplexers using multimode interference couplers

Novel designs for phased-array wavelength-division multiplexers based on self-imaging properties of multimode interference (MMI) couplers are presented. These devices, which operate on N equally spaced wavelength channels, consist of two MMI couplers connected by an array of N monomode waveguides. The MMI couplers function as power splitters/combiners, and the waveguide array is the dispersive element. The excellent characteristics of MMI couplers offer the possibility of designing small-size devices with low loss and with high uniformity among different channels. A general theoretical formulation for an N-channel multiplexer is presented, and a simple procedure for finding an optimum set of lengths for the array guides is given. We show that these multiplexers can function as N x N wavelength-selective interconnecting components. The simulated performance of three variations of a five-channel device, designed in a rib waveguide system, is given. It is demonstrated that sidelobes in the multiplexer spectral response can be suppressed by weighting the power samples in the array waveguides through appropriate design of a nonuniform MMI power splitter.     

Scanning near-field optical microscopy as a tool for the characterization of multimode interference devices

We report the scanning near-field optical microscopy (SNOM) characterization of a 4 x 4 multimode interference (MMI) device working at a wavelength of 1.55 microm and designed for astronomical signal recombination. A comprehensive analysis of the mapped propagating field is presented. We compare SNOM measurements with beam-propagation-method simulations and thus are able to determine the MMI structure's refractive-index contrast and show that the measured value is higher than the expected value. Further investigation allows us to demonstrate that good care must be taken with the refractive-index profile used in simulation when one deals with low-index contrast structures. We show evidence that a step-index contrast is not suitable for adequate simulation of our structure and present a model that permits good agreement between measured and simulated propagating fields.     

Synthesis of Hadamard transformers by use of multimode interference optical waveguides

We propose a synthesis method of optical Hadamard transformer using multimode interference (MMI) couplers. By using the signal transfer matrix of 2 x 2, 4 x 4, and 8 x 8 MMI couplers, we show that sum and difference units of input signals can be synthesized. An interchange unit of two signals can also be synthesized. One synthesis method of Hadamard transformers is a combination of only 2 x 2 units, and the other is a combination of N x N(N > or = 4) units as well as 2 x 2 units. The design examples of operation units are shown, and the size and the output power of Hadamard transformers are estimated.     

Tunable power splitter based on an electro-optic multimode interference device

Power splitters based on multimode interference (MMI) devices that offer the possibility of dynamically tuning the power-splitting ratio using electro-optic (EO) polymers are presented. The so-called 1 x 2 electro-optic MMI (EO-MMI) is demonstrated to provide a tuning range of 6 dB at approximately 54 V as theoretically predicted. Also a method is discussed to reduce the driving voltage by generating multiple beats, which provide 15 V for a tunable range of 10 dB for r(33)=15 pm/V at wavelength 1.55 microm.     

Two-dimensional model for three-dimensional index-guided multimode plasmonic waveguides and the design of ultrasmall multimode interference splitters

We demonstrate that a three-dimensional (3D) index-guided multimode plasmonic waveguide can be approximated to a two-dimensional (2D) lossy slab waveguide by using an effective-index method. It is found that this 2D approximation is more accurate when the width of the multimode waveguide increases. Such a 2D approximation can be used for a quicker and more efficient design of complicated multimode plasmonic devices. 1 x N ultrasmall multimode interference splitters based on multimode surface plasmon waveguides are designed by using this 2D model and the designs are validated with a 3D finite-difference time-domain method.     

Lensed-taper launching coupler for small-bore, infrared hollow fibers

A new type of launching coupler for small-bore, hollow fibers, consisting of a lens and a tapered hollow waveguide, is proposed to increase the alignment tolerance between an input laser beam and small bore fibers. First, we designed the structural dimensions of the coupler by using a ray-tracing method. Then, a series of experiments employing tapered hollow waveguides made of Pyrex glass was performed to investigate the effectiveness of the new coupler. It is shown that the coupler has a high efficiency with attenuation of around 0.5 dB, especially when the inside of the taper section is coated with a polymer and silver film. In addition, we also show that the coupler has great tolerance for the transverse displacement of a waveguide axis, which gives a 0.1-dB loss increase for a 300-mum displacement.     

Effect of external index of refraction on multimode fiber couplers

The dependence of the performance of fused-taper multimode fiber couplers on the refractive index of the material surrounding the taper region has been investigated both theoretically and experimentally. It has been identified that for a 2 × 2 multimode fiber coupler there is a range of output-power-coupling ratios for which the effect of the external refractive index is negligible. When the coupler is tapered beyond this region, the performance becomes dependent on the external index of refraction and lossy. To analyze the multimode coupler-loss mechanism, we develop a two-dimensional ray-optics model that incorporates trapped cladding-mode loss and core-mode loss through frustrated total internal reflection.

Computer-simulation results support the experimental observations. Related issues such as coupler fabrication and packaging are also discussed.

     

Demonstration of an optical isolator by use of a nonreciprocal phase shift

The experimental study of an optical isolator by use of a nonreciprocal phase shift is demonstrated. The isolator has an optical interferometer composed of tapered couplers, nonreciprocal phase shifters, and a reciprocal phase shifter. The isolator, designed for a 1.55-mum wavelength, was fabricated to investigate the characteristics of each component. The branching and coupling characteristics of the tapered coupler were measured. The nonreciprocal and reciprocal phase shifts were also evaluated. By applying an external magnetic field to the interferometer, we confirmed the nonreciprocal phase shift in the interferometric isolator.     

In-line broadband 270 degrees (3lambda/4) chevron four-reflection wave retarders

The net differential phase shift Delta(t) introduced between the orthogonal p and s linear polarizations after four successive total internal reflections inside an in-line chevron dual-Fresnel-rhomb retarder is a function of the first internal angle of incidence phi and prism refractive index n. Retardance of 3lambda/4 (i.e., Delta(t)=270 degrees) is achieved with minimum angular sensitivity when phi=45 degrees and n=1.900822. Several optical glasses with this refractive index are identified. For Schott glass SF66 the deviation of Delta(t) from 270 degrees is < or = 4 degrees over a wavelength range of 0.55 < or = lambda < or = 1.1 microm in the visible and near-IR spectrum. For a SiC prism, whose totally reflecting surfaces are coated with an optically thick MgF(2) film, Delta(t)=270 degrees at two wavelengths: lambda(1)=0.707 microm and lambda(2)=4.129 microm. This coated prism has a maximum retardance error of approximately 5 degrees over > three octaves (0.5 to 4.5 microm) in the visible, near-, and mid-IR spectral range. Another mid-IR 3lambda/4 retarder uses a Si prism, which is coated by an optically thick silicon oxynitride film of the proper composition, to achieve retardance that differs from 270 degrees by < 0.5 degrees over the 3-5 microm spectral range.     

Millimetre-wave monopulse antenna incorporating substrate integrated waveguide phase shifter

A compact planar E-plane monopulse antenna is proposed and realised at 36.5 GHz for millimetre-wave radar or direction-finding system application, which is built on the substrate integrated waveguide (SIW) technology. A phase shifter having a non-uniform SIW width configuration is self-consistently made of a periodically embedded via array and it can achieve good performances over a relative broad bandwidth. A 180deg directional coupler incorporating this proposed phase shifter structure is developed in the design of an integrated feeding network for the monopulse antenna. Four elements array based on the configuration of antipodal linearly tapered slot antenna is designed as radiator, offering a pair of 'sum' and 'difference' beams along the long direction of the substrate. Measured gain of the 'sum' beam is higher than 16 dBi, whereas the zero depth of the 'difference' beam is lower than -38 dB. This type of monopulse antenna presents an excellent candidate in the development of intelligent millimetre-wave directional- finding system.     

Modeling and experimental observation of an on-chip two-dimensional far-field interference pattern

In this paper, we model and experimentally observe the far-field radiation produced by interfering beams propagating in two-dimensional (2D) slab waveguides. Using a transmission-line analogy, we compare the 2D propagation with standard three-dimensional (3D) far-field representations and derive the 2D conditions for using standard far-field approximations. Then we test our theoretical results by experimentally observing the 2D far-field pattern produced by a 1×3 multimode interference (MMI) coupler on a silicon nanomembrane. The MMI outputs are connected to a slab silicon waveguide, and the far field is observed at the edge of the silicon slab. This represents the observation of 2D far-field pattern produced by an array of on-chip radiators.     

Electro-optic multimode interference device using organic materials

We present experimental results verifying the optical robustness of a 1 x 1 multimode interference (MMI) device that is directly butt coupled with optical fibers at 70 degrees C for 1050 h and discuss the gradual increase of polarization dependent loss. Based on this structure, an electro-optic (EO) MMI waveguide device that can control the output optical power by using an electrode structure located directly on top of the multimode is presented. As a proof of principle, we demonstrate the switching operation of the EO-MMI device using commercially available chromophore as the active EO material.     

Rectangular characteristic gratings for waveguide input and output coupling

Normal-incidence planar-optical waveguide-imbedded phase gratings of finite aperture width and length are analyzed with Svidzinskii's (Sov. J. Quantum Electron. 10, 1103 (1980)] two-dimensional Braggdiffraction theory. Svidzinskii's characteristic-grating equations are adapted for the rectangulargrating case, and an overlap integral is used to extend the theory to account for the mode structure of the waveguide. The combined theory is used to optimize the throughput of a system composed of an input grating coupler, a waveguide, and an output grating coupler for both the highly multimode (thickwaveguide) and the few-mode (thin-waveguide) cases.     

Design of aperiodic multilayer structures for attosecond pulses in the extreme ultraviolet

The design of aperiodic reflecting multilayer (ML) structures for attosecond physics in the extreme ultraviolet spectral region is presented. An optimization procedure based on "evolutive strategy" has been developed in order to get coating structures reflecting high photon fluxes in ultrashort duration pulses. The MLs are designed for a specific (75-105 eV) spectral interval with suitable reflectance and phase characteristics, in particular high total spectral reflectivity coupled with very wide bandwidth, spectral phase compensation, and amplitude reshaping. Furthermore, to take into account manufacturing tolerances, solutions stable with respect to random layer thickness variations are selected. To test the reliability of the proposed design procedure, examples of Mo/Si ML structures designed to reflect ultrashort pulses with different amplitude profiles and phase behavior are considered. The performances of the various structures are analyzed.     

Evaluation of some GRIN fiber parameters and the associated fraction mode loss due to mechanically induced optical anisotropy

Some of the optical parameters of the bent multimode graded-index (GRIN) optical fiber in terms of indices of refraction, where the bending stresses broke the radial symmetry, are evaluated by use of multiple-beam Fizeau fringes. The variation of the index difference between the cladding index and core index in both the compression and tensile fiber regions is measured. The accuracy of measuring the index is +/- 1 x 10(-4). The spatial resolution of the method is 1.39 microm. Evaluation of the acceptance angle, the numerical aperture, and the V number profiles of the bent fiber from the interference pattern at both sides of the bent fiber are presented. The fraction of the mode number lost has been evaluated. The method was used to study the influence of compression on diminishing the index difference that leads to a dissipation of energy and a considerable mode loss. It is obvious from the experimental data that the change of the index difference due to bending strongly affects the fraction of propagating mode number, especially at the small radii of curvature. Ignoring the variation of the index difference we evaluating the number of propagated modes number leads to an insufficient determination of the mode loss. It subsequently leads to an incorrect determination of the mode dispersion and the interface loss in bent GRIN fibers. The study confirms that the deviation of the guide axis from straightness with the radius of curvature of less than 1 cm could lead to a significant fraction mode loss.     

Embedded centrosymmetric multilayer stacks as complete-transmission quarter-wave and half-wave retarders under conditions of frustrated total internal reflection

A centrosymmetric multilayer stack of two transparent materials, which is embedded in a high-index prism, can function as a complete-transmission quarter-wave or half-wave retarder (QWR or HWR) under conditions of frustrated total internal reflection. The multilayer consists of a high-index center layer sandwiched between two identical low-index films with high-index-low-index bilayers repeated on both sides of the central trilayer, maintaining the symmetry of the entire stack and constituting a QWR (Delta(t)=90 degrees or 270 degrees ) or HWR (Delta(t)=180 degrees ) in transmission. A QWR design at wavelength lambda=1.55 microm is presented that employs an 11-layer stack of Si and SiO(2) thin films, which is embedded in a GaP cube prism. The intensity transmittances for the p and s polarizations remain >99% and Delta(t) deviates from 90 degrees by <+/-3 degrees over a 100 nm spectral bandwidth (1.5< or =lambda< or =1.6 microm), and by < or =+/-7 degrees over an internal field view of +/-1 degrees (incidence angle 44 degrees < or = phi(0)< or =46 degrees inside the prism). An HWR design at lambda=1.55 microm employs seven layers of Si and SiO(2) thin films embedded in a Si cube, has an average transmittance >93%, and Delta(t) that differs from 180 degrees by <+/-0.3 degrees over a 100 nm bandwidth (1.5< or =lambda< or =1.6 microm) and by <+/-17 degrees over an internal field view of +/-1 degree . The sensitivity of these devices to film-thickness errors is also considered.     

A low-makeup beveled tip capillary electrophoresis /electrospray ionization mass spectrometry interface for micellar electrokinetic chromatography and nonvolatile buffer capillary electrophoresis

A robust interface has been developed for interfacing micellar electrokinetic chromatography (MEKC) and nonvolatile buffer capillary electrophoresis (CE) to electrospray ionization mass spectrometry (ESI-MS). The interface consists of two parallel capillaries for separation (50 microm i.d. x 155 microm o.d.) and makeup (50 microm i.d. x 155 microm o.d.) housed within a larger capillary (530 microm i.d. x 690 microm o.d.). The capillaries terminate in a single tapered tip having a beveled edge. The use of a tapered beveled edge results in a greater tip orifice diameter (75 microm) than in a previous design from our laboratory (25 microm) that used a flat tip. While maintaining a similar optimum flow rate and consequently similar sample dilution, a 75-microm beveled emitter is more rugged than a 25-microm flat tip. Furthermore, the incorporation of a sheath liquid capillary allows the compositions of the final spray solution to be controlled. The application of this novel CE/ESI-MS interface was demonstrated for MEKC using mixtures of triazines (positive ion mode) and phenols (negative ion mode). The ability to perform CE/ESI-MS using a nonvolatile buffer was demonstrated by the analysis of gangliosides with a buffer consisting of 40 mM borate and 20 mM alpha-cyclodextrin.